The invention relates to a switching device with a power FET which is provided to switch a current flowing through an inductive load, which is coupled between an operating voltage terminal and the drain terminal of the power FET, and to the gate terminal of which a series resistor is connected whose terminal facing away from the gate terminal can be connected to a control voltage for the power FET, with a free-wheeling diode connected in parallel to the inductive load, and where the parasitic drain source diode included in every FET may also be used for this purpose.
Such arrangements are known and are useful in motor vehicle applications, for instance. It is well known that the free-wheeling diode is employed to protect the FET against a voltage surge that occurs due to the inductance when switching off the power field-effect transistor (FET). If the FET is switched on again after such a short time after having been switched off that injection charges are to be found in the region of the depletion layer of the free-wheeling diode, or in other words if the recovery time has not yet expired, then the free-wheeling diode is not yet capable of blocking when voltage is applied in the reverse direction. In this case, therefore, a very high current flows through the FET for a short period of time that can endanger the FET and the diode and/or other components. It is well known that this problem can be overcome by using a driver circuit that has been specially designed for such cases so that when switching on it slowly takes the FET out of the non-conducting zone into the fully conducting zone. Such state-of-the-art circuits are, however, very elaborate. The very high current flowing through the free-wheeling diode and the FET can also cause electromagnetic disturbances which require elaborate interference suppression measures in order to obtain the necessary electromagnetic compatibility (EMC) characteristics.
A driver circuit for a power FET in an inductively loaded load circuit with a free-wheeling diode is already known from DE-OS 40 13 997 A1 and DE-OS 44 13 546 A1, where a controllable current source enables the voltage to build up accordingly across the free-wheeling diode. Furthermore, means of measuring the voltage in the load circuit are required in accordance with DE-OS 40 13 997 A1, these means being described in DE-OS 44 28 674 as a feedback path. Such circuit arrangements are expensive.
Instead of the special driver circuits mentioned above according to the state of the art, the problem can also or additionally be solved up to a specific slope rate by faster (and hence more expensive) free-wheeling diodes which change particularly quickly from the conductive state to the blocked state.
Furthermore, it should be noted in this context that the FET should produce as little power loss as possible to allow it to be switched as fast as possible from the non-conductive to the conductive state and vice versa. The driver circuit should have as low a resistance as possible and thus the gate terminal should not have too great a series resistance connected to it on the input side. The low-resistance driver results in steep switching signal edges. Especially if the switching device is to be operated with high frequencies (for example, 10 kHz or more), as in the present case, when the FET is to regulate the current through the inductive load by means of pulse width modulation (PWM), it might be necessary to have steeper switching signal edges which in turn introduce problems relating to the above-mentioned reverse recovery time of the free-wheeling diode.